The present invention relates generally to a remote detection and imaging system. More particularly, the invention relates to a light detection and ranging (LIDAR) system using dual detectors that allows remote three-dimensional imaging of underwater objects, or other objects hidden by a partially transmissive medium.
LIDAR technology relies upon the time-of-flight of a light pulse, typically a short burst of laser energy, to and from an object to determine the range (location) of the object relative to the source of the light pulse. Using LIDAR technology, for example, it is possible to detect a submerged object from an airborne platform. The underwater objects remotely detected by a LIDAR system are normally categorized as being either small (on the order of 1 meter) and shallow, e.g., a moored mine; or large (greater than 10 m) and deep, e.g., a submarine.
Recently, there has been a great interest in using LIDAR systems not only to detect the presence of an underwater object, but also to provide an image of the detected object so that the object can be classified and/or identified. If a LIDAR system is to be used to efficiently identify and classify an object, it will normally be necessary to generate a high resolution image of the outer surface of the object. This, in turn, requires that the object depth be known, i.e., that the round trip time of a light pulse to and from the object be known (or otherwise be determinable), so that an appropriate detector or camera can be enabled (gated ON) at just that time, thereby receiving a return pulse of light from just the object, and not from some other object or medium that is above or below the object. Unfortunately, until the object has been detected, the approximate round trip time of the light pulse is not known, and the LIDAR system cannot be efficiently used for imaging. Hence, heretofore it has generally been necessary to first use the LIDAR system to hunt for the object, and to second (once the object has been found) provide a sufficiently high resolution image to identify and/or classify the object.
In hunting for an object using a LIDAR system, a first laser pulse is generated from a location above a target area (generally a body of water), and a suitable camera (or other detector) is shuttered ON at a time that corresponds to a pulse returning from a given depth or "slice" of the target volume. If nothing is detected at that depth, then another pulse is generated and the camera is shuttered ON at a time that corresponds to a pulse returning from a slightly different depth. In this way, i.e., by generating multiple pulses and gating ON a detector at slightly different times for each return pulse, the target volume is examined "slice" by "slice" until an object, if any, is located within the target volume.
Unfortunately, the above-described approach requires the generation of multiple laser pulses, and thus requires a great deal of laser power. Further, the described approach provides an extremely slow scan rate, or the rate at which a given target volume can be examined for the presence of an object. This is because the target volume must be examined slice by slice, and each slice requires the generation of a laser pulse and the looking for a return pulse at a particular shutter ON time relative to the laser pulse generation. Hence, what is needed is a more efficient LIDAR system that utilizes less power and provides a faster scan rate.
In principle, the foregoing deficiencies can be addressed by generating a single laser pulse and employing multiple gated cameras as detectors. Each camera is equipped with a separate receiver optical system, and all of the cameras are adjusted to image the same portion of the exposed surface of the target volume (which is typically the surface of a body of water). Assuming N gated cameras, the gate timing among the N gated cameras is adjusted so that with the transmission of a single laser pulse, N different gate images are generated, with each image corresponding to a separate "slice" of the target volume.
Using multiple gated cameras in this manner has not been reported previously, to applicants' knowledge, and thus represents one embodiment of applicants' invention. However, using multiple gated cameras is not a preferred embodiment because it requires very complex and relatively large signal processing equipment having relatively high power requirements. Further, using multiple gated cameras limits the maximum receiver optics aperture, and because of the massive on-board signal processing requirements, occupies a large portion of the available airborne packaging space. Hence, what is needed is a LIDAR system that is not only simple in terms of processing capabilities, but which is also small and light weight, consumes little power, and is reliable in its operation.
As taught in U.S. Pat. No. 4,862,257, issued to Ulich, it is known in the art to use an imaging LIDAR system for both the detection and classification of submerged objects. In the '257 patent, a system is described wherein a short pulse of laser light is projected down toward the surface of the water and to any objects that may be submerged below the surface of the water. At least one, and preferably two, range gated intensified charge coupled device (CCD) camera(s) are electronically shuttered (gated ON) during a time interval which coincides with the round trip propagation time of the laser pulse to and from the object. The resulting gated images are displayed on a CRT. The gated image from one CCD camera is timed to coincide with the depth of the object. The gated image from the other CCD camera is timed to coincide with the shadow of the object against the backscattered light. These two images are then subtracted to improve the detectability of the object.
Unfortunately, the approach proposed in the '257 patent requires that initial detection of the object be performed so that the cameras can be shuttered at an appropriate time. This difficulty can be overcome in principle by modifying the teachings of the '257 patent by setting one deep gate at the greatest depth an object is expected to be encountered. Objects shallower than this depth can be detected based on the shadow signature alone. However, this modified approach discards the direct reflection from the object surface, thereby reducing the detection signal-to-noise ratio (SNR). It also limits the detection SNR for all shallower object depths to the lower SNR value associated with a single deep gate.
The '257 patent also teaches the generation and transmission of laser pulses at a prescribed frequency, e.g., 15 Hz, and the use of a glint detector to detect the return pulse from the water surface. The glint return triggers a basic timing reference point from which a precise time delay is measured and/or adjusted in order to gate ON a CCD camera at the appropriate time. Because of the time delays involved (&gt;1 ms), it turns out that the CCD camera is actually triggered on the previously detected glint pulse. See col. 7, lines 10-11, of the '257 patent. Use of the glint return pulse allows the system, once calibrated, to operate independent of the distance from the water surface.
Unfortunately, however, use of a glint pulse in the manner described in the '257 patent still requires some "hunting" for the exact location (depth) of the target, so that the appropriate delays may be generated to gate ON the CCD camera at the appropriate time to see the object.
It is clear, therefore, that significant benefits in detection SNR, laser power utilization efficiency, search rate, and signal processing load will accrue if the foregoing difficulties can be alleviated.